The advice is that Microsoft is working with Activision to re-instate the digital pre-order on Xbox One and in the mean time you can secure your pre-order via retailers:

We do not comment on the specific terms of our arrangements with any of our publishing partners, however we are actively engaged with Activision to return ‘Call of Duty: Black Ops III’ to the Xbox Store. In the meantime, fans can pre-order ‘Call of Duty: Black Ops III’ for Xbox One or Xbox 360 from any of a host of retail partners here.

The original statement that was released this morning from Activision was:

Activision did not remove the digital version of Call of Duty: Black Ops III from the Xbox Games Store. Black Ops III is ready for launch on November 6th and is the most anticipated game of the year. Black Ops III is available for pre-order or pre-purchase through our other retail partners worldwide.

This means that the removal of the digital pre-order was made by Microsoft, for what reason we still don’t know. The good news though is if you are super keen on the digital download just sit tight as it will be back soon.

Director and Editor in Chief of Real Game Media.
I began my gaming obsession back on the Commodore 64 when I was very young and haven't stopped gaming since.
As for a bit about me, if I'm not gaming I'm raising and training reptiles!
I have a strong belief that we have a responsibility to shape the mind sets of the youth entering the gaming industry through our own actions and the examples we set, and at RGM we put a strong focus on ensuring that we set the best example we can.